This invention relates to laundry machines. In particular it relates to laundry machine drain pump control, power supplies, electronically commutated permanent magnet motor drives and motor braking during spin dry cycles.
Most laundry machine drain pumps use an AC induction motor or synchronous motor, operating off the mains frequency, which is typically 50 or 60 Hz. This fixed frequency, along with the mains voltage, determines the design of the pump, and hence the flow rate and maximum head height that can be obtained.
There are numerous disadvantages of a fixed mains-frequency pump motor. These are: (a) the starting torque is typically low, making the pump susceptible to being blocked, (b) separate pump designs are needed for countries having differing mains voltage and frequency, (c) the noise level during xe2x80x9cventilationxe2x80x9d is high, particularly when running at a supply frequency of 60 Hz, (d) the torque varies with the mains voltage and (e) there is no feedback to indicate if the pump is faulty.
By the nature of the washing process debris, including strands of textile, is removed from the wash load and flushed into the sump of the washing machine. This debris is then pumped out along with the dirty wash water. Occasionally the pump impeller will catch on the debris, or will start accumulating textile strands and then stop pumping.
Problems of this nature have been solved in other industries such as sewage handling, by having powerful motors driving pumps, with large clearances and large flow passages. Home appliances, however, do not have the space available to utilise large flow passages. One small volume solution in use is the vortex impeller, where the impeller has a shroud around it that eliminates the jamming point between vane end and the xe2x80x98cutwaterxe2x80x99. Unfortunately this pump has the disadvantage that it can still become blocked by bundles of fibres that become twisted xe2x80x9cropesxe2x80x9d in the centre of the vortex.
Traditionally a laundry washing machine is turned off by employing one of two methods. In the first power to the washer is isolated using an expensive mains rated switch which de-energises it and makes power consumption negligible. The switch must be capable of carrying and breaking the total current required at the required voltage. In the second method the machine is made to appear it has been switched off by removing all power on indications on its control panel. In reality the machine""s power usage is the same when it appears to be switched off as when it is switched on. These methods have the respective disadvantages that either a high current and/or voltage rated switch is required or power is consumed unnecessarily when the laundry machine is not in use.
The use of permanent magnet rotor three winding stator motors with electronic commutation of the windings is known. A common way of driving an electronically commutated three phase motor is to configure it in an H-Bridge between two dc rails, as shown in FIG. 5. The six power transistors Q1 to Q6 energise the motor windings by switching in a particular pattern. It is possible to have a high level of control over the speed and torque of the motor if feedback on the position of the rotor is available to the drive circuit logic which controls the switching pattern. The transistors are switched on by applying a voltage, usually 15V, from gate to source. Because the source voltage varies from 0V to the DC rail voltage, some technique is required to get the signal from the driving logic, which is referenced to 0V, to the high side gate which is referenced to the phase output (A, B, and C).
Common techniques are to use pulse transformers, purpose-designed ICs such as the IR2111, or optocouplers. A typical prior art circuit which employs an optocoupler for driving the high side transistor is shown in FIG. 6. When transistor Q2 switches off, noise is generated which tends to pull the optocoupler off. A logic inverter q4 ensures that turn off to the optocoupler means turn off of Q1. Since Q1 should always be off during turn off of Q2, correct operation of the circuit is not compromised. The disadvantage of this conventional optocoupler drive design is that high quality and therefore high cost devices are required.
During laundry machine spin dry cycles the spin tub is rotated at relatively high speed. At the termination of the spin cycle it is desirable to brake the motor to minimise spin tub run down speed. Conventionally this has been achieved by connecting resistors across the motor windings. This technique has the disadvantage that relatively high power resistors are required and these add to the cost of the laundry machine.
It is therefore an object of the present invention to provide a laundry machine having drain pump control, a power supply, a motor drive, and a motor braking system which overcomes the respective disadvantages outlined above.
In a first aspect the invention consists in a method of operating a drain pump in a laundry washing machine during the drain phase of the wash cycle to reduce pump blockages characterised in that:
(a) the pump is started and run for a first period of time,
(b) the pump is stopped for a second period of time which is less than 10% of said first period of time, and
(c) steps (a) and (b) are repeated for the duration of said drain phase.
In a second aspect the invention consists in a method of braking a top loading laundry washing machine spin tub wherein said spin tub is driven by an electronically commutated DC motor, wherein the commutation devices are connected to a DC power supply and have free-wheeling diodes connected in parallel therewith, and wherein said washing machine includes other components having inductive windings,
characterised in that:
commutation of power to the motor windings is terminated,
the voltage of the DC power supply is monitored,
and when the DC power supply voltage exceeds a pre-determined value the inductive winding of an unused component in said machine is connected across said DC power supply until the DC power supply voltage reduces below said predetermined value.
In a third aspect the invention consists in a method of powering on and off a laundry washing machine where power is consumed in the form of direct current using a switched mode power supply of the type described in Australian Patent 651408 characterised in that:
an active switching device connected between the base of the lower switch in the motor bridge drive used as part of said switched mode power supply and the lower voltage rail is switched on by a latching circuit to cause the DC power supplies in said machine to be disabled,
the latching circuit comprises a capacitor charged from the high voltage rail for said laundry machine motor in parallel with a transistor biased from the switched mode power supply low voltage rail, and
a push button normally off switch connected in parallel with said capacitor is used to discharge said capacitor to disable said latching circuit to thereby enable the laundry machine DC power supplies.
In a fourth aspect the invention consists in an optocoupler drive circuit for the high side of a totem pole power transistor pair in a bridge circuit for commutating a motor characterised in that the optocoupler does not use a logic inverter on the output side and the optocoupler transistor is controlled to limit the speed of turn off and thus the speed of turn on of the upper power transistor in the totem pole.
In a fifth aspect the invention consists in a laundry washing machine including a wash bowl drain pump characterised in that:
said pump is driven by a variable speed motor which in turn is driven by a variable frequency pulse width modulated inverter,
and the pump motor frequency is decreased at low bowl water levels to reduce the effects of ventilation.